In certain fields, such as the field of PC board assembly, small amounts of viscous materials must be applied accurately to a substrate. Such viscous materials have a viscosity greater than about 50 centipoise and include, for example, adhesives, solder flux, solder paste, solder mask, grease, oil, encapsulants, potting compounds, epoxies, dye attach pastes, silicones and RTV.
Manufacturers of PC boards use processes including screening, pin transfer and various other dispensing systems and methods for applying viscous materials to a PC board. Dispensing systems may include either a syringe dispenser or an ON/OFF dispenser valve. Syringe dispensers are used as contact dispensers actuated with pneumatic mechanisms, electro-mechanical mechanisms or positive displacement valves. The tip of a syringe dispenser is placed very close to the substrate, for example, at a distance of 0.005 inches for a very small droplet and a distance of about 0.060 inches for a larger droplet. The viscous material is pushed out of the syringe tip and contacts the substrate while it is still connected to the syringe tip. Syringe style contact dispensers have certain desirable attributes. However, one of the main drawbacks with syringe dispensers relates to their relatively slow operating speed. That is, it is difficult to dispense more than about ten dots of viscous material per second with a syringe dispenser. Stringing of viscous material away from the syringe tip is also a problem that reduces accuracy and quality.
Dispenser valves, which can operate as non-contact ON/OFF dispensers, exhibit certain advantages relative to other dispensing systems and methods. For example, dispenser valves can more quickly dispense a number of minute drops of viscous material on a PC board. This is generally due to elimination of z-axis movement which is necessary with contact dispensers. Also, dispenser valves can reduce or eliminate stringing problems and other problems associated with contact dispensing methods since the dispenser nozzle may be maintained at a greater distance from the substrate. Dispenser valves also allow a greater amount of viscous material to be cleanly and accurately dispensed at a single location by discharging more than one drop at the location.
Despite the various improvements in this field, including those pertaining to contact and non-contact dispensers, additional improvements are necessary to more accurately control the dispensed amount of viscous material. In this regard, variations in the amount of dispensed viscous material can occur due to variations in the characteristics of the viscous material itself or, for example, other machine or dispensing system tolerances. In other words, although the various operating parameters of the dispensing system may be maintained constant, variations may occur in the amount of dispensed material due to other outside factors. For at least these reasons, it would be desirable to provide systems and methods for more accurately controlling the amount of viscous material discharged from a dispenser.